Process for the chemical stabilization of paper and product



Patented Mar. 4 10, 1936 PATENT OFFICE PROCESS FOR THE CHEMICAL.STABILIZA- TION OF PAPER AND PRODUCT Otto J. Schierholtz, Toronto,Ontario, Canada,

assignor to Qntario Research Foundation, Toronto, Ontario, Canada, acorporation of Ontario, Canada No Drawing. Application December 10,1934,

Serial No. 756,896

11 Olaims.

This invention relates to the stabilization of paper whereby it isrendered non tarnishing to metals, suc

as silver, bronze, etc., with which it is broug t into contact and inertwith respect to inks and colors, etc., applied to the paper, andmoredurable with age.

The invention is designed principally for th treatment of paper, such aswall paper, newspaper, wrapping paper, etc., made from chemical pulp andmechanical pulp which contains acid or acid-forming bodies. Unsizedpaper, such as filter paper, which has been purified or refined to ahigh degree, is non-tarnishing to metals and relatively stable withoutthe treatmen The stabilizing process consists essentially in introducingan alkaline earth metal bicarbonate,

such as strontium. bicarbonate, calcium bicarbonate or bariumbicarbonate into. the paper after the paper web has been formed andpreferably after it is'sized. Many sizing processes, such as thewell-known alum-rosin size, require acidity and/or give rise to acidityin the paper, and the stabilizing treatment should, therefore, followsuch sizing treatments in order not to interfere with the sizingtreatment and also to take care of any acidity resultingfrom the sizingtreatment. v

The alkaline earth metal bicarbonate may be introduced into the paper inseveral ways,-as

relatively small amounts and does not, and is not intended to, serve tofill the paper, although some slight filling action may incidentallyoccur due to reversion of the bicarbonate to the normal carbonate or tothe use of a solution of bicarbonate containing the normal carbonate insuspension.-

The bicarbonate is of such character and is applied in such limitedamount that it does not iniure thepaper in any way, i. e. attack andweaken the cellulose fiber, or produce discoloration, or impair thefinish, or interfere with the sizing of the paper, or interfere with theuse of the paper, e. gas a wrapping paper for foods.

As a result of the treatment the paper is left and may be applied to thefinished paper or as a it is passing over the drying rolls.

The instability of paper and its tarnishing properties generally areattributed to or associated with its acidity, i. e. to the presence inthe paper of acid-acting substances or substances which'tend to developacidity as the paper ages. This acidity or tendency to develop aciditymay be inherent in the cellulosic fibrous material used, as in the caseof mechanical pulp, or it may result from the chemical treatmentemployed for the production of the pulp, or it may result from thesizing of the paper with materials such as alum, or a conibination ofthese causes. 'However, the treatment is not to be regarded as beingmerely a chemical neutralization of the paper because it has been foundthat paper may be neutralized by treatment with, for instance, causticalkali and filled with acid-neutralizing bodies, such as calciumcarbonate, without accomplishing a stabilization thereof, such as isaccomplished by the process of the present invention. Paper may moreoverbe rendered non-tarnishing to metals ,by incorporating materials such aslead.

paper, i. e. in, the form of a solution, to the formed sheet, and if asizing treatment requiring' the presence of acid or resulting in thepresence of acid is used, the treatment must be applied after thesizing. Materials such as calcium carbonate, introduced into the paperat an earlier stage 'in the formation of the paper are not effective,probably because they react with the acid or acid-forming constituentsof the paper too slowly and incompletely.

The instability of paper may be shown and tested in various ways, forinstance by testing its strength after an accelerated aging, or bytesting its tarnishing action on metals under accelerated conditions.The following method has been found to be most convenient for testpurposes. A specimen of paper is coated with a suspension of gilt powderin a starch gum paste and placed in a closed glass vessel containingwater so as to maintain a high humidity and allowed to stand usually forabout 48 hours at room temperature. The gilt or metal powder is an alloyof copper, zinc and aluminum in about the following proportions: copper,91.2%; zinc 6.1%; and aluminum, 1.7%. The starch paste is of the typecommonly used in the paper industry and is composed of starch, borax,glucose, phenol and water. If the paper specimen is unstable the giltwill show a definite tarnish ing.

While this test determines only the tarnishing properties of the paper,it has been found that it is also a fairly accurate indication of thestability of the paper because ordinarily a paper which will tarnish isalso unstable. However, instability, that is, tendency of the paper todeteriorate, particularly in strength, with age may not be directlyproportional to the tarnishing properties of the paper and this may bedetermined by the so-called oven test. This test is not as delicate asthe tarnishing test, but has been found to show that paper which hasbeen stabilized in accordance with the present invention generallysufiers only about one-half as much in loss or strength as untreatedpapers, other than those of the type of high grade filter paper, whichare inherently non-tarnishing and stable. ti The following exampleillustrates the invenon.

An aqueous solution of barium bicarbonate is prepared by bubbling carbondioxide through an aqueous suspension of finely divided barium carbonateuntil a suitable quantity of the barium carbonate has been converted tothe bicarbonate and dissolved, i. e. to a concentration of about 0.15 to0.20 percent of barium bicarbonate, ii. the carbonation is carried outin the cold and without pressure. A higher concentration may be obtainedby carbonation under pressure. The suspension of barium carbonate maycontain any desired excess thereof over that necessary to give thedesired concentration of barium bicarbonate. The resulting solution ofbarium bicarbonate containing barium carbonate in suspension may beapplied to the paper directly, or it may be first filtered to remove theundissolved barium carbonate. The paper to be stabilized is immersed inthe solution for a length of time determined by tests, which time mayvary from about 5 seconds, i. e. just sufilcient to wet the paper up toabout 2 minutes. The time required depends upon the concentration of thebarium bicarbonate solution and the character of the paper, and sincethe cause of tarnishing and instability are not exactly known, the timerequired cannot be accurately determined by chemical analysis of thepaper; However, some indication as to the time of immersion required maybe obtained by observing the tarnishing properties of the paper in theabove described tarnishing test. As a result of numerousexperiments Ihave found that a relatively unstable paper, such as one made fromground or mechanical pulp, requires an impregnation with about 40 poundsof barium bicarbonate per ton of paper. Other less unstable papersrequire correspondingly smaller amounts.- After the impregnation of thepaper with the barium bicarbonate solution, it is dried, preferablywithout the application of high temperature.

As an alternative to the above described procedure involving immersionof the paper in the barium bicarbonate solution, the paper may besprayed on or between the drying rolls of the paper-making machine withthe barium bicarbonate solution, or the paper may be impregnated with asolution of barium hydroxideither by immersion or spraying, and thentreated with carbon dioxid either in gaseous form or in solution toconvert the barium hydroxid to the carbonate and bicarbonate. I

Although tarnishing and instability in paper appear to be associatedwith acidity, and my treatment has been found to result in bringing thepH value of the paper to approximately 6.5

or higher, nevertheless my treatment is not merely a chemicalneutralization of the acidity of the paper, because, as has been statedabove, a sample of paper may be neutralized to the same extent withsodium hydroxid without rendering it chemically stable as determined bythe gilt tarnishing test described above.

The described treatment with barium bicarbonate confers upon cheappaper, such as ground wood paper, some of the properties of ahighlypurified paper such as filter paper. The treatment does notintroduce anything into the paper which is objectionable in connectionwith ordinary commercial uses of paper. The treatment is inexpensive andrenders many cheap papers, which otherwise would be unstable, entirelysatisfactory for use as-gilt or'decorated wall paper, gilt printingpaper and the like;

and aside from the tarnishing properties the paper is rendered resistantto 88 18. The treatment of the paper does not interfere with the normalsizing process. 4

The treatment of paper with the other alkaline earth -metalbicarbonates, strontiumbicarbonate and calcium bicarbonate, may beidentical with the procedure described above in connection with the useof barium bicarbonate, excepting that the solubility of thesebicarbonates must be taken into consideration in determining thequantity of solution to be applied. Mixtures of the bicarbonates of twoor more of the alkahne earth metals may of course be used and thepresence of small amounts of other materials in the impregnatingsolution, such as sodium carbonate or bicarbonate, is not excluded,although, as is apparent, the impregnating solution should be free ofany substance having a detrimental action on the paper.

I claim:

,1. Process of stabilizing paper and rendering it non-tarnishing whichcomprises impregnating the paper with an. alkaline earth metalbicarbonate in aqueous solution.

2. Process as defined in claim 1 in which the solution of the alkalineearth metal bicarbonate contains the normal carbonate of. said metal insuspension.

3. Process as defined in claim 1 in which the impregnation isaccomplished by'treatlng the impregnation with alkaline earth metalbicarbonate solution is applied to the paper after it has been sized.

5. Process as defined in claim 1 in which the impregnation with alkalineearth metal bicarbonate solution is applied to the paper while it isbeing dried.

6. Process as defined in claim 1 in which the paper is immersed in thealkaline earth metal bicarbonate solution for from about 5 seconds toabout 2 minutes.

'7. Process as defined in claim 1 in which the alkaline earth metalbicarbonate solution contains about 0.15 to 0.20% of barium bicarbonate.

8. Process as defined in claim 1' in which the alkaline earth metalbicarbonate solution contains calcium bicarbonate.

9. Process as defined in claim 1 in which the alkaline earth metalbicarbonate solution contains strontium bicarbonate.

10. Process as defined in claim 1 in which the paper is impregnated withabout 40 pounds of barium bicarbonate per ton of paper.

11. Paper which has been stabilized and rendered non-tarnishing byimpregnation with an alkaline earth metal bicarbonate.

OTTO J. SCIHERHOLTZ.

